This invention relates to communication systems employing antenna arrays and has particular application to multicarrier communications systems such as those employing orthogonal frequency division multiplexing (OFDM) modulation.
Antenna arrays have a plurality of antennas used to communicate radio frequency signals through wireless communication links. Antenna arrays provide improved performance relative to a single antenna by providing a better antenna pattern for a coverage area.
Even with an antenna array to provide an improved antenna pattern, signals communicated between communication devices are subject to interference. Buildings, hills and other objects produce multi-path wave propagation and communication devices and energy sources introduce noise, resulting in errors in the signals communicated between communication devices.
To reduce these errors, techniques have been developed to optimise the received path of a communication device employing an antenna array. By varying the weight of the signals detected by each of the individual antennas in the array, it is possible to vary the antenna pattern to better detect signals from a particular direction or to arrange for non-destructive combination of multi-path signals. These techniques adjust the weights of the antenna array signals to maximise the receive path gain by measuring the output of a receiver.
Other techniques are known whereby optimum weights are provided for the transmit path. For example, Applicant""s co-pending application no. GB-A-2313261 discloses a method of weighting a transmit path of a communication station xe2x80x98Axe2x80x99 which is equipped with an antenna array. A method includes the steps of transmitting reference signals from each antenna in the array to a communications station B and calculating at station B weighting information based on a comparison of the incoming reference signals with stored reference signals. The calculated weighting information is then transmitted from station B to station A whereupon a controller in station A adjusts the antenna weights based on the received weighting information.
Thus, a feedback mechanism as described above can be implemented to provide optimised transmission settings for each antenna comprising the array.
However, applying such a scheme to a wide-band multicarrier system (e.g., OFDM) which operates in frequency selective channels, results in a large overhead. A large overhead may prove unacceptable in certain systems.
It is further known that for high data transmission rates, the channel frequency response becomes frequency selective i.e. the phase and amplitude characteristics vary with frequency within the total band occupied by a transmitted signal. Therefore, in systems employing transmitting antenna arrays and multicarrier modulation, sub-carrier frequency-dependent weights have to be used, thereby increasing the overhead still further.
In the particular case of frequency division duplex (FDD) where up-link and down-link transmit on different carrier frequencies, a feedback weighting scheme is required. However, again, large amounts of overhead are required in order to transmit the reference signals in one direction and subsequently, the frequency-dependent weighting information in the other direction for every separate subcarrier (in a multicarrier FDD system).
This invention aims to provide a method and apparatus for reducing this overhead whilst maintaining most of the achievable gain.
In a first aspect, the present invention comprises: apparatus for generating antenna weights for a transmit path of a first communication device, said first communication device including an antenna array, in which the apparatus includes, in a second communication device, a receiver for receiving multi-carrier signals comprising a plurality of sub-carriers transmitted from the antenna array over a plurality of sub-bands, means for measuring a parameter of the received signals for each sub-carrier comprising each sub-band, means for identifying at least one sub-band including a sub-carrier whose measured parameter meets a predetermined criterion, a transmitter for transmitting to the first communication device a request for a reference signal in said identified sub-band, and calculating means for calculating antenna weights for each sub-carrier included in said identified sub-band from an analysis of said reference signal.
In a second aspect, the present invention comprises a method for generating antenna weights for a transmit path of a first communication device, said first communication device including an antenna array, in which the method includes the steps of;
in a second communications device,
receiving multi-carrier signals comprising a plurality of sub-carriers transmitted from the antenna array over a plurality of sub-bands,
measuring a parameter of the received signals for each sub-carrier comprising each sub-band,
identifying at least one sub-band including a sub-carrier whose measured parameter meets a predetermined criterion,
transmitting to the first communication device a request for a reference signal in the identified sub-band,
receiving from the first communication device a reference signal in the identified sub-band,
and calculating antenna weights for each sub-carrier included in said identified sub-band by analysis of the reference signal.
The calculated weights may be transmitted from the second communication device to the first communication device whereupon weight circuits in the first communication device may be utilised to set the antenna weights for each sub-carder to optimise the transmit path.
Hence, the second communication device generates feedback information in order to maximise the quality of the signal it receives from the first communication device.
The second communication device may be provided with an antenna array or a single antenna.
The multi-carrier signals transmitted over a plurality of sub-bands may comprise OFDM signals and the reference signals may comprise pilot symbols.
The measured parameter may be, for example, received signal quality and those sub-carriers with received quality falling below a pre-set threshold may be identified and the sub-band to which they belong, probed by transmission of the requested reference signals. Antenna weights may be calculated by one of several appropriate known techniques. For example, the reference signals may be correlated with a local stored reference to give an estimate of the gain and phase of the transmit path.
By probing only those sub-bands where received signal quality could be improved, the total transmission overhead is reduced.
As antenna weight values for only selected sub-bands may be calculated and fed back to the first communication device, the computational overhead is kept low and the transmission overhead is reduced still further.
Different criteria of signal quality can be used to derive the optimum weights depending on the system embodiment e.g. received power or signal to interference ratio among multiple communication units if they all share the same frequency.
In order to limit the number of bits of information representing the antenna weight values to be fed back to the first communication device, compression algorithms may be used. Another option for minimising the amount of information fed back may involve determining sub-optimal weights.
Means for identifying sub-bands (a sub-band being defined as comprising a group of sub-carriers which are all affected by the channel in substantially the same way) may comprise means for estimating the channel response of the transmit path.
In one embodiment, the second communication device is provided with means for estimating a coherence time for the transmit path. (Coherence time being defined as a length of time during which the properties of a channel are constat). These parameters can be either programmed into the second communication device or periodically estimated from the signals received from the first communication device. Knowing the coherence time, the second communication device can set a preferred update rate. This update rate specifies how often the second communication device requests the reference signals to be sent and how often it feeds back the antenna weights. The update rate may be set at an optimised value taking into account the coherence time, and the available capacity of the communications link between the first and second communication devices.
Consequently, the second communication device may incorporate at least one timer circuit, each being associated with each sub-band, and set in order to avoid re-probing recently probed sub-bands before transmit path characteristics are expected to change. The countdown settings of each timer circuit may be derived from the coherence time.
In the context of OFDM communication systems, the invention provides the advantage of requiring less transmitter power for the same quality of service. This can also result in reduced interference, more efficient frequency re-use and/or range increases.
Further, the average number of subcarriers transmitted at each antenna is statistically reduced. Hence peak to average power requirements for transmitter amplifier can be reduced.
More capacity in one direction of the communications link between first and second communication devices can be obtained but at the expense of capacity in the other direction.
In systems incorporating spatial diversity, specifications for coding or inter-lacing can be relaxed.
Significant improvements can be achieved with very little extra overhead for low delay spread environments which create significantly wide notches.
One particularly useful application of the invention is in wireless local area networks in which servers are equipped with multiple transmitting antennas and the clients use single antenna transceivers.
A further application of the invention is to broadband transmissions which could be a combination of OFDM and other types of multiple access coding such as code division multiple access.